Windshield wiper system with drive arm

ABSTRACT

This invention relates to windshield wiper system which utilizes a flexible drive arm for withstanding bending loads applied to a wiper. The drive arm may be made of a pull-molded composite material having a relatively low modulus of elasticity and a relatively high elongation factor. The flexible arm twists in the presence a bending load and undergoes rapidly progressing elastic buckling when the bending load exceeds a predetermined amount.

BACKGROUND OF THE INVENTION

This invention relates to a windshield wiper system and, moreparticularly, to a windshield wiper system which utilizes areciprocating, flexible arm for driving a windshield wiper.

An example of a prior art windshield wiper drive link and system isshown in Buchanan et al., U.S. Pat. No. 6,148,470, which is incorporatedherein by reference and made a part hereof. A windshield wiper system,as taught therein, is particularly useful for driving in snow or in mud,under conditions wherein an accumulation of foreign material may cause asudden blockage of the wiper block. When this happens, the windshieldwiper motor may generate a momentarily large driving torque in anattempt to overcome the blockage. That in turn may cause permanentdamage to one or more components of the wiper system.

A flexible arm, as taught in Buchanan et al., reduces the risk of suchdamage by constructing the wiper drive arm from a material whichtolerates compression loads up to a predetermined limit. Below thatlimit, known as the critical buckling load limit, the drive arm simplycompresses by an amount proportional to the force of the load. However,upon reaching the critical buckling load limit, the arm gives way bypronounced elastic buckling. The buckling effectively prevents anyfurther increase in the load being applied to wiper system components,and does so without permanent injury to the drive arm. Once the blockagehas been removed, manually or otherwise, the flexible arm simply popsback into its original configuration.

As further taught in Buchanan et. al. U.S. Pat. No. 6,148,470, theflexible drive arm may be interposed between a drive motor and a pair ofdrive plates. The drive plates in turn apply drive torques cooperativelyto a pair of wiper blades. The flexible drive arm preferably is madefrom a composite material of a type described in Table I of the patent.Four specific materials are taught, including a molded glass laminate, amolded epoxy resin, and two pull-molded polyesters having oriented glassfibers.

As further disclosed in Buchanan et al, the flexible drive arm may begenerally elongated and generally rectangular in cross-section. Thepatent teaches that the flexible drive arm could have othercross-sectional geometries, such as elliptical or circular, and in onedescribed configuration could have a length of at least about 250 mm.Notches could be fabricated in the flexible drive arm in order to adjustthe bending stress at which elastic buckling occurs. The patent observesthat a suitable flexible drive arm should have a design strength suchthat buckling is not expected to occur in the face of a compression loadless than about 30 percent greater than the normally expected maximumrunning load for a comparably sized steel or rigid link that does notflex.

The prior art also includes a windshield wiper for an aircraft, asshown, for example in Rogers et. al (U.S. Pat. No. 4,318,201). Thatpatent teaches a flexible drive arm for a windshield wiper wherein thecross-section varies from end to end in order to control the onset ofelastic buckling. The Rogers patent also discloses the use of a glassfiber composite for construction of a flexible drive arm for awindshield wiper.

SUMMARY OF THE INVENTION

This invention improves the performance of a windshield wiper byproviding it with a flexible drive arm supported by a hollow tubeextending from a motor to a wiper arm. The tube preferably has anormally unstressed sideward curvature for relaxation along a windshieldwhen the wiper axis rests on a curved portion thereof. The cross-sectionof the hollow tube has an off-center shear center. As the wiper axismoves to a flat portion of the windshield, the contact of the wiperagainst the windshield generates a sidewardly directed bending(unbending) force which stresses and straightens out the drive arm alonga cross-sectional width. The straightening of the drive arm sets upinternal bending stresses which flatten the hollow tube therebyprogressively decreasing the moment of inertia about the longitudinalaxis. Transverse blocking results in elastic buckling when thesidewardly applied bending force reaches a predetermined level. That inturn relieves the stress on the windshield drive motor and wipercomponents when the wiper system or arm becomes blocked.

In one aspect, this invention comprises a drive arm for a windshieldwiper comprising a member having a preselected cross-section and acurvature extending laterally from the longitudinal axis, the tubularmember defining a wiper axis for connection of a wiper thereto and alsodefining a motor axis for connection of a drive motor thereto and thetubular member generally bending about the longitudinal axis when abending force is applied thereto.

In another aspect, this invention comprises a windshield wiper systemcomprising a drive motor, a drive arm coupled to the drive motor and awiper blade also coupled to the drive arm for wiping a windshield whenthe drive motor is energized; the drive arm being made of a compositematerial and being generally curved and comprising cross-sections thatvary along its length.

Other objects and advantages of the invention will be apparent from thefollowing description, the accompanying drawings and the appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sketch of two cooperatively installed windshield wipersystems, one of which is stalled by blockage material;

FIG. 2 is an isometric drawing of a flexible drive arm;

FIG. 3 is a drawing of a cross-section of a first embodiment of aflexible drive arm, taken at a lightly stressed station thereof;

FIG. 4 is a drawing of a cross-section of a second embodiment of aflexible drive arm;

FIG. 5 is a drawing of a cross-section of the flexible drive armembodiment of FIG. 3, taken at a heavily stressed station thereof;

FIG. 6 is a schematic illustration of braiding of pull-molded compositematerial about the surface of a flexible drive arm;

FIG. 7 is a plan view of a drive arm in accordance with an embodiment ofthe invention;

FIG. 8 is a side view of the drive arm shown in FIG. 7, illustrating thecurvature of the side arm;

FIG. 9 is a cross-sectional view taken along the line 9-9 in FIG. 8;

FIG. 10 is a cross-sectional view taken along the line 10-10 in FIG. 8;

FIG. 11 is a cross-sectional view taken along the line 11-11 in FIG. 8;

FIG. 12 is a cross-sectional view taken along the line 12-12 in FIG. 8;

FIG. 13 is a view of the drive arm having one end mounted to a shaft ofa motor, with the second end having a wiper blade mounted thereon and inoperative engagement with a windshield;

FIG. 14 is a cross-sectional view taken along the line 14-14 in FIG. 13;

FIG. 15 is a cross-sectional view taken along the line 15-15 in FIG. 13;

FIG. 16 is a cross-sectional view taken along the line 16-16 in FIG. 13;

FIG. 17 is a cross-sectional view taken along the line 17-17 in FIG. 13;

FIG. 18 is another view of the drive arm as it engages debris on awindshield;

FIG. 19 is a sectional view taken along the line 19-19 in FIG. 18;

FIG. 20 is a cross-sectional view taken along the line 20-20 in FIG. 18;

FIG. 21 is cross-sectional view taken along the line 21-21 in FIG. 18;

FIG. 22 is another view of the wiper arm shown in FIG. 18 as a motorcontinues to drive the arm;

FIG. 23 is a cross-sectional view taken along the line 23-23 in FIG. 22;

FIG. 24 is a cross-sectional view taken along the line 24-24 in FIG. 22;

FIG. 25 is a cross-sectional view taken along the line 25-25 in FIG. 22;

FIG. 26 is another view of the wiper arm illustrated in FIG. 22;

FIG. 27 is a cross-sectional view taken along the line 27-27 in FIG. 26;

FIG. 28 is a cross-sectional view taken along the line 28-28 in FIG. 26;

FIG. 29 is a cross-sectional view taken along the line 29-29 in FIG. 26;

FIG. 30 is a view illustrating a tube in a mold prior to inflation orenlargement;

FIG. 31 is a view similar to FIG. 30 where the tube has been inflated tomold the material to a shape which will define the drive arm;

FIG. 32 is a view illustrating various directional components forfacilitating an understanding the Moment of Inertia to be calculated asdescribed;

FIG. 33 is a view of another embodiment of the invention;

FIG. 34 is a cross-sectional view taken along the line 34-34 in FIG. 33;

FIG. 35 is a cross-sectional view taken along the line 35-25 in FIG. 33;

FIG. 36 is a cross-sectional view taken along the line 36-36 in FIG. 33;

FIG. 37 is a cross-sectional view taken along the line 37-37 in FIG. 33;

FIG. 38 is a cross-sectional view taken along the line 38-38 in FIG. 33;and

FIG. 39 is a cross-sectional view taken along the line 39-39 in FIG. 39.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 1, there are illustrated two windshield wipersystems, 10, 10, each of which has a wiper 14, connected to a flexibledrive arm 12 at a pivotal joint 16. Wipers 14, 14 each comprise a wiperblade (not illustrated), suitably supported by its associated pivotaljoint 16. For this purpose, the wipers 14, 14 may be fitted with spines(also not illustrated) assembled in well known whiffletree or flat bladearrangements. A pair of drive motors 18, 18 cause flexible drive arms12, 12 to carry wipers 14, 14 across windshield 20 in reversing arc-likepaths 24, 24, so as to remove debris therefrom. For ease ofillustration, the motors 18, 18 are shown coupled to the arms 12, 12,but it should be understood that drive linkage (not shown) may beemployed to couple the arms 12, 12 to a single motor 18 or multiplemotors 18, 18.

Still referring again to FIG. 1, the flexible drive arm 12 on the righthand side of the windshield 20 is shown to be working against arelatively unyielding pack 22 of foreign matter or debris. This producesa relatively high stress which tends to be transferred to the associatedmotor 18. In accordance with this invention, wear and tear on the motoris reduced by twisting the flexible drive arm to reduce the stiffnessthereof and sharply achieve elastic buckling upon occurrence of thepredetermined bending force. The principal structural component of aflexible drive arm 12 is a hollow tube 32, manufactured from a compositematerial, as hereinafter described. It should also be understood thatthe arm 12 may be solid as shown in the embodiment illustrated in FIGS.33-39.

The geometry of a tube 32 is illustrated in FIG. 2. Preferably, tube 32is fitted with an elastomeric hose 47, as shown in cross-section in FIG.3. Tube 32 is characterized by a cross-section having an off-centershear center 37. Tube 32 is secured to motor axis 26 by a stamping orcasting (not illustrated) and extends from motor axis 26 to a wiper axis28 in an initial direction indicated by a longitudinal axis 30 asillustrated in FIG. 2. Wiper 14 is attached to tube 32 along wiper axis28. Wiping loads are transferred from wiper 14 to tube 32 along thataxis. As further shown in FIG. 2, tube 32 has a pronounced sidewardcurvature which carries it laterally away from longitudinal axis 30. Thetotal amount of this curvature is indicated on FIG. 2 by the angle α.That is the non-stressed, rest configuration of tube 32, where nobending force is transmitted from wiper 14 to tube 32. When the arm 12is installed in operative relationship to a windshield, the arm 12flattens and reduces the moment of inertia in the “out-of-plane”bending. The arm 12 experiences lateral bending load as the arm 12sweeps across the windshield 20.

In the rest condition shown in FIG. 2, wiper 14 rests lightly againstthe curving contour of the windshield 20. However, when wiper 14traverses a substantially planar windshield region, the windshield 20reacts against wiper 14, thereby creating a bending force, which istransmitted along wiper axis 28 to tube 32. That produces a bendingforce, F, which stresses tube 32, causing flexible drive arm 12 tostraighten out in the direction of longitudinal axis 30. The bendingforce acting in the plane of the angle may be calculated from thefollowing equation:

$\begin{matrix}{F = {\frac{3{EI}\;\delta}{L^{3}} = {KS}}} \\{{{where}\text{:}\mspace{14mu} K} = \frac{3{EI}}{L^{3}}}\end{matrix}$where: k is the spring constant of the flexible drive arm

-   -   E is the modulus of elasticity of the flexible drive arm    -   I is the moment of inertia about the major axis    -   δ is the deflection in the direction of the major axis    -   L is the length of the flexible drive arm.

The value of the moment of inertia depends upon the position and thedirection of a reference axis about which the moment of inertia iscalculated. For example, FIG. 32 depicts a rectangular cross-section 83having a height h and a base b. Assume that this rectangularcross-section is made up of elemental areas, dA, having directionalcomponents dX and dY and that the Moment of Inertia, I_(x), is to becalculated about the x-axis. The value of I_(x) is given by theequation;

I_(x) = ∫y²𝕕Awhich can be expanded to:

$I_{x} = {\int_{\frac{- h}{2}}^{\frac{h}{2}}{{y^{2}\left\lbrack {\int_{o}^{b}{\mathbb{d}x}} \right\rbrack}{\mathbb{d}y}}}$or $I_{x} = {b{\int_{\frac{- h}{2}}^{\frac{h}{2}}{y^{2}{\mathbb{d}y}}}}$This  gives: $\begin{matrix}{I_{x} = {{{\frac{b}{3}y^{3}}|_{\frac{- h}{2}}^{\frac{h}{2}}} = {\frac{b}{3}\left\lbrack {\left( {h/2} \right)^{3} - \left( {{- h}/2} \right)^{3}} \right\rbrack}}} \\{= {{\frac{b}{3}\left\lbrack {\frac{h^{3}}{8} + \frac{h^{3}}{8}} \right\rbrack} = \frac{{bh}^{3}}{12}}}\end{matrix}$

Consequently, flexible drive arm 12 is sufficiently stiff to carry abending force which varies in proportion to the minor axis length. Asflexible drive arm 12 bends toward longitudinal axis 30, tube 32generates a shear flow causing a bending stress that flattens tube 32about its shear center 37. The twist angle, so produced, is indicated bythe Greek letter β in FIG. 5. Twisting of tube 32 tends to flatten outthe cross-section thereof, as illustrated in FIGS. 22-28, which in turncauses a substantial reduction in the moment of inertia, I. This reducesthe stiffness of tube 32, as well as the force calculated by the abovenoted equation, so that elastic buckling occurs rapidly upon occurrenceof the predetermined force. It will be understood that the location ofshear center 37 is shown only approximately. The actual position issituated at a point such that a hypothetical shear load, equivalent tothe actual distributed shear load, would produce a twist, β, whendirected therethrough. Most preferably, the cross-section varies alongthe length of tube 30.

In the embodiment described, the arm 12 comprises a slenderness ratio,L/r of at least 50, but not more than 600,

-   -   where: L is the length of the arm 12; and    -   “r” is the least radius of gyration of the cross-section        (I=ar²), where I is the moment of Inertia and a is the area of        the cross-section.

In prior art wiping systems of the cantilever beam type the drive arm isoftentimes shaped such that aerodynamic wind forces of increasing speedtend to press the arm into the glass with lower intensity. Also, withsuch prior art systems the arm tip force normally increases at the tipas the beam is deflected. The present invention compensates for suchincreases by providing a beam cross-section having a moment of inertiaaffording a substantially constant tip force through the workingdeflection. In the preferred embodiment, flexible drive arm 12 has anoff-center shear center 37 which reduces arm twisting due to torsionalloads about wiper axis 28. In one preferred embodiment the off-centershear center may appear as a ‘smile’ or upwardly curved (as viewed inFIG. 2) cross-section (See FIGS. 3-5). The material has high elongationproperties and will allow for major deformation without breaking. Afrozen blade might twist the structure and the arm flex out of plane,breaking loose the ice.

In a typical prior art wiping system the arm would deflect 0-3 inches or75 mm. The deflection is caused by the rise and fall of the arm duringthe wiping action. In some cases there is no elastic buckling. Anotherembodiment is where the arm 12 is a one piece solid or tubularconstruction that is generally U-shaped in cross-section. Thiscross-sectional shape is similar to a cross-sectional shape of a steelcarpenter's rule. This embodiment produces the desired elastic buckling.

The drawing of FIG. 2 includes a series of lines representing spacedstations along a flexible drive arm 12. FIGS. 3 and 5 illustrate thecross-sections thereof appearing at stations 32 and 34 respectively.These cross-sections decrease in scale and also flatten down as flexibledrive arm 12 approaches station 32 from the direction of motor axis 26.This cross-sectional flattening is quite reminiscent of the snappingaction of a sidewardly bowed steel rule when extended beyond a certaincritical length and is due in part to the relative lengths of minor axis83 and major axis 85.

As illustrated in FIGS. 3 and 5, a flexible drive arm 12 may comprise athin elastomeric hose 45 encased within a glass/fiber composite tube 33.Hose 45 provides a passage for supplying washer fluid to the wipingblade (not illustrated). More preferably, however, a flexible drive arm12 has a configuration 50, as illustrated in FIG. 4. This particularembodiment features a fluid supply hose 47 encased within a tube 51 andsupported by a foam core 52. Preferably, drive arms 12, 12 aremanufactured from a fiber-reinforced plastic material, produced by awell known process called “pull-molding”. Broad background teachingsregarding pull-molding may be found by reference to U.S. Pat. No.6,253,411 B1 (Aichele et al.), the disclosure of which is herebyincorporated herein. This invention generally follows prior artteachings, such may be found in the patent produces a pull-molded strandof glass/plastic composite. It should understood that the strands may beglass, carbon or other suitable fiber. That strand (not illustratedherein) is stored on a suitable reel until required. At that time alarge batch of flexible drive arms may be produced in a joined,end-to-end, arrangement, withdrawing pull-molded composite material fromthe reeled strand, as required. Individual drive arms 12 may be sawedoff from the linked arrangements at such time as may be convenient. Ithas been found particularly convenient to store the still-joinedflexible drive arms on a large reel and to separate them just prior toshipment from the factory. It will be appreciated that the entireprocess could be consolidated at a single site, but it is feasible toparcel out parts thereof to separate contractors.

FIGS. 33-39 illustrates another embodiment of the invention. Notice thearm 13 is a solid uniaxial pull-molded wiper arm with diecastterminations 61 and 62 which are crimped onto the ends thereof as shown.It should be appreciated that the end or fitting 61 comprises an opening63 for mounting onto a drive motor 18 (FIG. 1). The fitting 62 may be ashaft or post for receiving a wiper blade 14 (FIG. 1). In the embodimentbeing described, the post 62 may be situated in an opening 65 and thenriveted onto the drive arm 13. In the embodiment being described, thisarm 13 is a right hand wiper arm which would be situated on the righthand wiper motor 18 (as viewed in FIG. 1). Note the cross-sectionaltransitions (FIGS. 34-39) of the pull-molded wiper arm 13.

The arm 13 comprises the following dimensions:

Dimensions Measurement (FIGS. 33-39) (mm) D₁  3.71 D₂  36.98 D₃  3.89D₄  36.2 D₅  6.13 D₆  23.11 D₇  7.78 D₈  18.24 D₉  8.2 D₁₀ 17.13 D₁₁12.53 D₁₂ 10.54 D₁₃ 55 D₁₄ 615 D₁₅ 640 D₁₆ 150 D₁₇ 110 D₁₈ 50 D₁₉ 240

It should be appreciated that the arm 13 may be of solid constructionwith fiber orientation, for example, the longitudinal direction of thearm 13. The arm 13 may comprise one or more of the features describedearlier herein relative to the other embodiments, such as a channel ortube through which wiper fluid may flow and the like.

Advantageously, the invention provides a lightweight, yet strong, drivearm having a relatively low modulus of elasticity and a relatively highelongation factor. Another advantage of the invention is that theflexible arm twists in the presence of a compressive load and undergoesrapidly progressing elastic buckling when the compressive load exceeds apredetermined amount such as when the wiper blade 14 (FIG. 1) encountersdebris on the windshield and the drive arm 12, 13, experiences a bendingforce that is greater than the maximum blade frictional force. Thesimple one-piece construction and elasticity of the drive arm 12, 13provides a lightweight, yet strong, drive arm that facilitates usingfewer number of components and parts which can become unusable ifoverfatigued.

In the preferred embodiment the overall process for making a flexibledrive arm includes the steps of:

-   -   1. Preparing a mold having an internal cavity characterized by        spaced cross-sections of generally one of the configurations        shown in FIGS. 1-39.    -   2. If desired, placing a tube in the cavity to provide a pathway        through drive arm 12, 13.    -   3. Placing flexible filler material inside the cavity to support        the hose and to provide a form for building the flexible drive        arm 12 or 13.    -   4. Foam cores are formed, for example, by inflating tube 51        (FIG. 31) to press foam against mold walls 53 a and 53 b (FIG.        31).    -   5. The successive cores are placed on a shipping roll (not        shown) for transport.    -   6. Fiber (glass, carbon, etcetera) is woven over each core as        described herein.    -   7. Woven cores are places on shipping roll (not shown) for        transport.    -   8. Shipping roll of woven, end-to-end cores are unrolled from        roll.    -   9. Woven cores are subject to resin bath to provide resin-coated        core.    -   10. Resin-coated core placed in mold.    -   11. Mold heats and cures resin-coated core.    -   12. Cured cores removed from molds.    -   13. Successive cores are cut apart.    -   14. End fittings are crimped onto ends of core to provide drive        arm 12.    -   15. Plastic or other material (not shown) may be optionally        over-molded over joints (not shown) between fittings and drive        arm 12.

Braiding of the pull-molded strand proceeds as illustrated in FIG. 6.During braiding, a plurality of strips of pull-molded working materialare fed to an automatic braiding machine which braids them into tubeshaving a generally crescent-shaped cross-section. At some locationsalong flexible drive arms 12, 12, the cross-sections may resemble thefamiliar “smiley face” commonly appearing on the contemporary Americanscene. Alternatively, they may be elliptical, or even concave on oneside with the opposing side being generally flat. The braiding machinecreates a triaxial braid at a braiding angle, y, which varies along thelength of the arm. This optimizes torsional stability and provides areduced stiffness perpendicular to the fibers, thereby minimizing armpressure variance as the arm tip rises and falls during the wiping ofthe windshield glass.

The tube walls comprise about twenty-one percent by weight of athermosetting polyester resin, seventy-five percent by weight of113E-Glass Roving and four percent by weight of a suitable filler. Itshould be appreciated that the percentages may vary from thosementioned, which are presented for purposes of illustration only. In itsunbraided state the working material has the following preferredmonotonic properties:

-   -   Elastic Modulus 43 GPa (6.2 Mpsi)    -   Ultimate strength of 11400 Mpa (165 ksi);    -   Strain at fracture=2.6%,    -   Specific Gravity=1.92

This composite material may be strained about 10 times as much as springsteel and is able to withstand a relatively large deflection withoutfracture. It is important that the ends of flexible drive arms 12, 12 beproperly terminated in order to deal with high stress concentrationsapplied along motor axis 26 and wiper axis 28. Bolt holes in the braidedmaterial along motor axis 26 and wiper axis 28 would fray and eventuallyfail, if made to carry the stress of ordinary nut and bolt attachments.Therefore this invention joins drive arms 12, 12 to other parts by meansof thermoplastic or thermoset overmolded, ductile steel, aluminum, zincor other metallic die cast stampings. A part to be joined to a drive arm12 is formed around and inside the pull-molded arm. Since the spacebetween the steel stamping and the fiberglass structure is small, any“Plastic Creep” effect is minimal, and the stress is transferredeffectively between parts.

The braid illustrated in FIG. 6 is a structure is similar to the weaveof a sock or a Chinese torture finger tube, only woven and placedcontinuously on a reel, perhaps over a foam core. It should beunderstood that the braid could be straight in a longitudinal directionalong its length or even straight in a vertical direction (as viewed inFIG. 6) or any combination thereof.

Note in FIG. 30 that the drive arm 12 could also be formed byalternative methods. For example, an elastometric tube 51 may be placedin a mold 53. Foam 55 or other suitable forming material is situatedaround tube 51, as illustrated in FIG. 30. The tube 51 is then expandedby gas, such as air or water against walls 53 a and 53 b of mold 53 (asshown in FIG. 31) to provide an arm 12. The arm 12 may then be processedwith one or more of the steps 3-12 described earlier herein. It isenvisioned that the tube 51 may then be deflated and subsequently usedto provide a passageway or channel for washer fluid through the arm 12.

FIGS. 7-24 illustrate further features of an embodiment of the inventionhaving the characteristics described.

FIG. 7 illustrates a plan or top view of the composite wiper arm asdescribed earlier herein relative to FIG. 2. Note the generally curvedshape illustrated in FIG. 8 which facilitates providing theaforementioned tip force which facilitates maintaining the wipers 14against the windshield 15. FIGS. 9-12 illustrate various cross-sectionalviews of the arm in an embodiment of the invention when the arm is atrest. These shapes are further illustrated in FIGS. 13-17.

FIGS. 18-24 illustrate a general change in the cross-sectional shapewhen the wipers engage the debris 22 (FIGS. 1 and 13). FIGS. 14-16illustrate the cross-sectional shape of the arm 12 when it first engagesthe debris 22.

As the motor 18 (FIG. 17) continues to drive the arm 12 in the directionof arrow A in FIG. 17, the arm experiences increased torque and beginsto twist in the manner illustrated and described herein. Note that theleading edge 12 a begins to move toward the debris 22 as shown and thetrailing edge moves away from the windshield as illustrated in FIG. 18.This slight twist and movement of the arm is also experienced at thecross-sectional areas illustrated in FIGS. 19 and 20.

FIG. 22 illustrates a more exaggerated twisting motion as the motor 18continues to attempt to drive the arm 14 in the direction of arrow A.Further twisting of the arm is illustrated and FIGS. 22-24 further showthe movement of the leading and trailing edges 12 a and 12 b,respectively. It is important to note, as illustrated in FIGS. 22 and 23and described earlier herein, that the cross-sectional shape of the armchanges from the generally curved shape shown, for example, in FIGS. 18and 19, to a generally flatter shape illustrated in FIGS. 22 and 23.This feature of the invention facilitates distributing the encounteredload forces and torque across the length of the arm 12 which in turnfacilitates accommodating the elastic buckling which would normallyseverally damage the wiper arms used in the past.

While the systems and methods herein described, and the forms ofapparatus for carrying these systems and methods into effect, constituteone embodiment of this invention, it is to be understood that theinvention is not limited to these precise methods and forms ofapparatus, and that changes may be made in either without departing fromthe scope of the invention, which is defined in the appended claims.

1. A drive arm for a windshield wiper system comprising: a tubularmember having a preselected cross-section and a curvature extendinglaterally from a longitudinal axis; said tubular member defining a wiperaxis for connection of a wiper thereto and also defining a motor axisfor connection of a drive motor thereto; said tubular member comprisinga first portion defining a first cross-sectional shape and a secondportion defining a second cross-sectional shape, wherein said firstcross-sectional shape is generally crescent shaped and said secondcross-sectional shape is non-crescent shaped; and said tubular membergenerally bending about said longitudinal axis and changing itscross-sectional shape when a bending force caused by an obstruction on awindshield is applied to the drive arm that causes said generallycrescent-shaped first portion to become generally flatter.
 2. The drivearm according to claim 1 wherein said tubular member experiences anelastic buckling when said bending force exceeds a predetermined amount.3. The drive arm as recited in claim 2 wherein said firstcross-sectional shape defines a width that generally increases when saidwiper blade encounters debris on said windshield and said drive armexperiences said bending force that is greater than a predeterminedforce.
 4. The drive arm as recited in claim 3 wherein said predeterminedforce is greater than the blade maximum frictional force.
 5. The drivearm as recited in claim 2 wherein said tubular member defining aninternal cavity extends along a longitudinal axis, with a curvatureextending laterally from said longitudinal axis.
 6. The drive armaccording to claim 5 wherein said tubular member experiences a localizedelastic buckling when said bending force exceeds a predetermined amount.7. The drive arm according to claim 5 wherein said wiper axis beingpositioned to cause a bending of said tubular member in a manner thatopposes said bending force.
 8. The drive arm according to claim 5wherein a cross-sectional width of said tubular member flattens out suchthat at least a portion of said cross-sectional shape is generally flat.9. The drive arm according to claim 1 wherein wiper axis beingpositioned to cause a flattening of said tubular member in a mannerwhich opposes said bending force.
 10. The drive arm according to claim 1wherein said preselected cross-section having an off-center shearcenter.
 11. The drive arm according to claim 1 wherein the secondcross-section shape is elliptical.
 12. The drive arm according to claim1 wherein said tubular member comprises a slenderness ratio (L/r) ofless than
 600. 13. The drive arm according to claim 1 wherein saidtubular member comprises a slenderness ratio (L/r) of greater than 50.14. The drive arm as recited in claim 1 further comprises: a wiper bladecoupled thereto at said wiper axis for wiping a windshield when saiddrive motor is energized; said drive arm being made of a compositematerial.
 15. The drive arm as recited in claim 14 wherein said secondcross-section shape defines a shape that is elliptical, or where or oneside of said drive arm is generally flat.
 16. The drive arm as recitedin claim 14 wherein a leading edge of said first cross-sectional shapemoves towards said windshield and a trailing edge of said firstcross-sectional shape moves away from said windshield when said wiperblade encounters debris on said windshield and said drive armexperiences said bending force that is greater than a predeterminedforce.
 17. The drive arm as recited in claim 16 wherein saidpredetermined force is greater than the blade maximum frictional force.18. The drive arm as recited in claim 14 wherein said drive arm twistswhen said wiper blade encounters debris on said windshield and saiddrive arm experiences said bending force that is greater than apredetermined force.